Wave energy is a concentrated form of renewable energy that comes from the friction between the water surface and the wind. The energy is built up by the wind on the open seas and then transported to locations closer to the shore, where it can be extracted with wave energy converters. Due to the high energy density of ocean waves, wave power is very area efficient and the average energy content changes more slowly and predictably compared to, for example, the wind. The resources are vast and can be harvested close to populated areas.
However, there are great challenges that must be solved before wave power can be commercially viable. Intermittent and highly fluctuating energy from the ocean waves must efficiently be converted into a steady output of electricity that is suitable for the power grid. Ocean waves have never ceasing variations in height, length, direction and time period (velocity) from wave to wave at a given sea state. A sea state is defined by the significant wave height (Hs) which is calculated from the average of the highest ⅓ of 100 waves in a row. The sea state will change slowly but largely over time; in storm conditions the average energy content in a sea state can be over 100 times higher than during normal conditions (annual average).
In order for a wave energy converter to capture energy efficiently, it should have sufficient length of stroke to follow the highest waves in the maximum wave condition it is designed for and intended to operate in, and it must be able to adjust to or handle changing sea levels and wave directions. Energy capture in a wave energy converter depends of the motion speed and force between an energy capturing device, such as a floating buoy, and a fixed or moving reference. If only one direction of motion can be used, e.g. the vertical motion, the motion speed will change from zero to a top level twice for every wave period. Looking at the vertical motion speed of the water particles in a wave, the motion speed is highest where the water particles pass through the average sea level and zero in crest and trough. In any given sea state the peak energy is found in the highest waves, statistically determined by Hs*1.8. The peak energy captured from the largest waves is in the order of 7-10 times higher compared to the average energy in any given sea state.
Converting the captured energy instantly is not efficient. The power will then fluctuate rapidly from zero up to the peak level which is not suitable from a conventional generator or the power grid to which it delivers the generated power. The generator must be sized to handle the peaks of energy which will lead to low utilization and low electrical efficiency with high heat generation as a consequence. The peak power can be distributed to speed and torque in the generator but neither one can be allowed to exceed a maximum value without damaging the generator. A higher speed will naturally lead to higher torque unless the damping of the generator is dynamically changed to compensate.
However, compensating the damping of the generator to reduce the peak torques will reduce the electrical efficiency even further. For this reason the peak torque/mechanical load will also reach high values as a consequence of the peak power being converted. An alternative is to limit the maximum instantaneous energy capture or spill excessive energy captured but this will reduce the utilization of all other parts of the wave energy converter by reducing the average power output.
To make possible efficient conversion of wave energy to electricity, as much energy as possible must be captured and then smoothed to a steady load on the power takeoff and generator with some kind of power smoothing device. In hydraulic power takeoffs it is common to use gas accumulators, but the gas pressure and thereby the hydraulic pressure in the hydraulic system increases exponentially with the level of energy stored in the accumulator, making it difficult to achieve sufficient power smoothing without very large size of the gas accumulator. To achieve a smooth power output matching the average energy from a given sea state, the capacity must be sufficient to smooth energy over several waves in a row. Two or three large waves may occur in a row after which number of smaller waves may occur. It is often said that every seventh wave is a large one and as said above, 100 waves in a row is measured to determine a sea state.
Wave power technologies have been developed for a long period of time but up to now it has not been shown how to design a wave energy converter that is able to efficiently convert the intermittent and highly fluctuating energy form ocean waves into a steady power output, as described above.
A frequent method of capturing the energy of water waves is to use the vertical movement of the water. Installations that use such technology are sometimes called “point absorbers”. One method of using the vertical movements comprises the use of a buoy having a bottom foundation and an anchor wheel. The bottom foundation is firmly positioned on the sea-floor and is connected to the buoy which follows the ocean surface, i.e. the wave movements. When the surface rises and thereby lifts the buoy, a motive force is created which is converted into a rotational movement by a driving bar connected between the foundation and the buoy or by a wire or chain which runs over an anchor wheel journalled for rotation at the buoy or in the foundation and which is at an opposite end connected to the foundation or the buoy, respectively. The motive force increases due to the increased motion speed of the waves when the wave height becomes higher. The rotation direction and speed of an anchor wheel, if such a wheel is used, is directly dependent on the vertical direction and motion speed of the waves. However, this is not optimal for coupling a conventional generator to the anchor wheel to produce electric energy.
In order to make a wave energy converter driving a conventional rotating generator efficient, the vertical movements of the waves must be converted into a unidirectional rotational movement, and the rotation speed of an electric generator connected to the transmission must be stabilized. In a device, as described above, using a driving bar, wire or chain, which is secured to the bottom of the sea or in a frame structure and which runs along or over an anchor wheel journalled in a buoy, this problem can partly be solved in the following way. When the buoy is lifted by a wave, a motive force over the anchor wheel is produced. Thereupon, when the wave falls, an anti-reverse mechanism is disengaged and the anchor wheel is rotated backwards by a counterweight. Then, the motive driving is only active during the rise of the wave and completely ceases when the wave sinks, this not being satisfactory.
Attempts have been made to reverse the rotation direction, so that an electric generator driven by the anchor wheel is driven by the counterweight in the same direction also when the wave sinks. It has also been attempted to reverse the rotation direction of the generator. However, changing the rotation direction of a mechanical transmission or of the generator twice in every wave period results in mechanical wear. Even though the rotation direction can be made unidirectional by the transmission, the rotation speed follows the speed of the vertical movement, this causing the speed and torque of the generator to vary according to the speed of the wave movements. This gives high fluctuations in the power output and torque load in the system and as a consequence also low efficiency and utilization of the generator since the generator has to be oversized to handle the peak loads. The power takeoff must also be oversized to handle the peak torques.
In order to make the motive force and rotation speed of a generator more even when using a mechanical transmission, multiple buoys can cooperate with each other, a phase shift then existing between the movements of the buoys. However, this only works optimally in the case where the buoys are evenly distributed over a wave period, which very seldom occurs since the length and the speed of the waves always vary.
Some of the basic disadvantages of the wave energy converters having the structure described above are eliminated or at least significantly reduced in the wave energy converters disclosed in the published International Patent Application No. WO 2009/105011. In such wave energy converters energy from water waves is in the common way, during parts of the movements of the water waves, absorbed for driving an electric generator. Part of the absorbed energy is temporarily accumulated or stored in some suitable mechanical way for driving the electric generator during other parts of the movements of the water waves. The driveshaft coupling of the movement of the water level and the mechanical energy storage to the electric generator is in a special mechanical way arranged for a unidirectional rotation with a constant torque and a constant rotation speed.